The chaotic listing of Facebook Inc. on the Nasdaq stock market last month was the latest example of how computer-trading systems can go haywire. The social network's botched IPO came hard on the heels of another embarrassing glitch. In March, BATS Global Markets, a computer-driven exchange, failed to list its own stock due to a software bug.

Mishaps such as these are damaging the confidence of small investors in the integrity and reliability of stock markets, critics say.

The root of retail investors' end of the love affair with the stock market can be traced back to May 6, 2010, the day of the now-infamous "flash crash."

A dark pool is an electronic platform where investors trade shares privately, away from more transparent stock exchanges. What do they have to hide? Lam Thuy Vo shines a light on dark pools.

In the space of a few, hair-raising minutes, a breakdown in the market triggered by failures in computer-trading systems across the country caused stocks to plunge about 10%. Despite extensive probes by regulators, the cause of that sudden crash remains a mystery.

The flash crash left a scar on the market. Investors have taken money out of the U.S. stock market funds in 17 of the 25 months since then, withdrawing a net $137 billion, according to Lipper.

Panic Ticks

Thomas Peterffy had seen it all. The Black Monday crash of October 19, 1987, the 1998 collapse of the giant hedge fund Long-Term Capital Management, the implosion of the dot-com bubble in 2000 and 2001, the credit crisis of 2008.

ENLARGE

Thomas Peterffy, founder and CEO of Interactive Brokers, has been voicing his opinion about the impact of high-frequency trading on the markets.
Jesse Neider for The Wall Street Journal

But what was unfolding on the afternoon of May 6, 2010 was different. This was fast. This was high-speed trading.

The founder of Interactive Brokers Group Inc. and Timber Hill, a sophisticated computer-driven trading operation, was monitoring the market from a private study on his luxurious estate in Greenwich, Conn.

Chaos was breaking out amid a burst of riots on the streets of Athens, Greece. Stocks had been on their heels all day. But now things were getting much worse. Down about 2% at 2:30 p.m. Eastern, the market had started to plunge rapidly.

Mr. Peterffy picked up the phone and called Timber Hill's trading desk, several miles away in downtown Greenwich.

"What the heck is happening?" he said.

"Don't know," a rattled trader replied.

"Well find out!" Peterffy shouted.

As the Timber Hill traders scrambled to find the cause of the problem, they started seeing a wave of "panic ticks" on their screens—warning signs indicating that their positions were moving so rapidly that they were risking big losses.

By 2:40 p.m., the number of panic ticks exploded. Mr. Peterffy called the trading desk again to see if anyone knew what was happening.

No one did.

Tradebot

In a bland, cube-shaped building on the outskirts of Kansas City, Dave Cummings watched from his corner office as the stock market unraveled like a ball of yarn.

ENLARGE

Dave Cummings, Tradebot's founder
Reuters

The founder of Tradebot Systems, one of the world's most advanced high-frequency trading operations, wasn't sure what to make of the downward draft. The heavy volume was scrambling trading systems, leading to disparities in prices quoted on various exchanges. The decline became so sharp that it made Mr. Cummings worry that it wasn't going to right itself.

Like many others, he worried that a "fat finger" mistake by a trader—Wall Street slang for someone who pressed the wrong button or put too many zeros into a sell order—had triggered a cascade that was turning into a vicious feed-back loop.

If there was an erroneous trade, that meant Tradebot's systems, which tracked all corners of the market for signals about future conditions, were operating on bad information. If Tradebot kept trading, it might spread the turmoil elsewhere, like a contagious virus.

Digitalized and Decimalized

As the stock market plunged, then Senator Ted Kaufman (D., Del.) was presiding as chair of the Senate. A wave of chatter rippled through the chamber as the senators, clicking on their handheld devices, stared in amazement at news of a major crash in the stock market.

Mr. Kaufman had been one of the fiercest critics of the computer-driven machines that had taken over the market. But he never thought he would see anything like what had just occurred.

Addressing his colleagues from the floor, he explained how the market had shifted from a floor-based system to one that was "digitalized and decimalized."

"People came into the market and began to develop these high-speed computers," he said. "Human beings were no longer doing the trading, computers were. They developed these algorithms. It ran automatically. It grew and grew. There is no way to know what is going on. No one knows what is happening in these exchanges when this trading is going on. We have a very dangerous situation."

Pools of Darkness

In the weeks and months following the flash crash, a fierce debate erupted over what had become of the stock market. Angry words were exchanged in the halls of Capitol Hill, on financial television shows and at trading firms in New York and Chicago.

Congress held panel discussions. The Securities and Exchange Commission grilled the previously unknown chieftains of the high-speed merchants, including Mr. Cummings of Tradebot and Mr. Peterffy of Timber Hill.

The complex, labyrinthine nature of the market vexed ordinary investors. Years ago, before the rise of electronic networks, most trading took place at the New York Stock Exchange and Nasdaq.

BY 2012, trading occurred in roughly seventy different venues, including giant hedge funds and banks. So-called "dark pools," private markets in which trading took place away from public exchanges such as the NYSE, accounted for more than 10% of all U.S. stock trades, according to Tabb Group.

As the markets slid into discrete pools of darkness, investors, too, had been left in the dark.

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